This invention relates to a raster distortion correction circuit and, more particularly, to such a circuit for correcting both pincushion distortion and trapezoidal distortion of a raster on a cathode ray tube.
In cathode ray tubes (CRT) of the type used in large-screen video displays, such as a television receiver, the so-called pincushion distortion of the raster on the CRT display screen often is present. This pincushion distortion is due to the fact that the CRT electron beam scans the display screen at a constant angular velocity but, since the screen is flat, the beam must move across a larger distance per time unit interval at the corners of the screen than at the center portion thereof. That is, if the trace of the beam is divided into equal angular increments as the beam scans from one side to the other of the viewing screen, the length of the beam from its deflection center to the screen is greater at the corner than at the center portion of the screen, resulting in a larger beam displacement for an angular increment at the corner of the screen than at the central portion of the screen. Of course, if the screen is curved such that the length of the beam from its deflection center to the screen is equal for all angular increments, the beam displacement at the screen is equal for all angular increments, regardless of whether the beam impinges at the corner or central portion thereof. Stated otherwise, since the beam has a constant deflection velocity, i.e., a constant angular velocity, its linear velocity across a flat screen is greater at the corners thereof than at the central portion. This difference in linear velocity, or displacement increment, results in the so-called pincushion distortion of the raster.
Various techniques have been proposed heretofore for correcting such pincushion raster distortion. For example, in one proposed technique, the power supply voltage for the horizontal deflection circuit of the CRT is varied, or modulated, by a parabolic-shaped signal having a frequency equal to the vertical deflection frequency of the CRT. Another technique for correcting pincushion distortion in a large-screen CRT, that is, in a CRT having a relatively large side-to-side beam deflection angle, is described in U.S. Pat. No. 3,968,402, assigned to the same assignee as the instant invention.
Another form of raster distortion in a CRT display is the so-called trapezoidal distortion. Trapezoidal distortion is due to the correcting apparatus used to correct for electron beam mislanding caused by the magnetic field of the earth. That is, it has been found that when a CRT display screen faces a particular compass direction, the earth's magnetic field affects the beam such that it does not land at the proper location on the CRT screen or on a shadow mask which may be used therewith. This beam mislanding is not uniform; rather, it is a function of the direction in which the CRT screen is facing. While correcting apparatus is known to correct for such beam mislanding, such apparatus results in the trapezoidal distortion of the raster. That is, typically, this correcting apparatus vertically displaces the effective deflection center of the beam so that with this "new" deflection center, the beam lands at the correct location. However, because of this displacement in the beam deflection center, the resultant raster exhibits a trapezoidal-shaped distortion.